updated docs

docs/docs/linear_algebra.xml

@@ -1067,7 +1067,13 @@
          <file>dlib/geometry.h</file>
          <spec_file link="true">dlib/geometry/vector_abstract.h</spec_file>
          <description>
-            This object represents a two or three dimensional vector.
+            This object represents a two or three dimensional vector.  
+
+            <p>If you
+            want to work with general N-dimensional column vectors then you
+            should the <a href="#matrix">matrix</a> object.  In particular, you
+            should usually use a matrix with this type:
+            <tt>dlib::matrix&lt;double,0,1&gt;</tt>.</p>
          </description>
       </component>